Ontogenetic development of dentition in the cave bear

Transcription

1 doi: /geologija Ontogenetic development of dentition in the cave bear Ontogenetski razvoj zobovja pri jamskem medvedu Irena Debeljak Ivan Rakovec Institute of Palaeontology, Scientific Research Centre of the Slovenian Academy of Sciences and Arts, Gosposka 13, SI-1000 Ljubljana, Slovenia Key words: Ursus spelaeus, Ursidae, dentition, jawbones, ontogenetic development, growth, age determination, sex dimorphism, mortality, Divje babe I Ključne besede:ursus spelaeus, Ursidae, zobovje, čeljustnice, ontogenetski razvoj, rast, določanje starosti, spolni dimorfizem, mortaliteta. Divje babe I Abstract In this contribution the ontogenetic development of dentition in the cave bear {Ursus spelaeus Rosenmüller & Heinroth) is described up to the age of about four years, when the animals became adult and the formation of their teeth was completed. The process of tooth growth and the replacement of deciduous teeth by permanent dentition took place in a similar way as with the present-day brown bear {Ursus arctos Linné). The teeth eruption sequence is the same in both species. The specimens of cave bear jawbones that served for this study were collected from the well-known Palaeolithic site Divje babe I (W Slovenia). Individual age estimations of these jawbones were made on the supposition that a certain ontogenetic stage in the development of cave bear dentition corresponds to an approximately equal age of an individual as with the brown bear. The growth of the jaw, however, was essentially faster in cubs of the cave bear We also presume that the secondary sex dimorphism in cave bears with males having larger jawbones than females was already expressed in the first year of life. With regard to some indices, especially after noting the obvious discordance between the expected and actually observed mortality profile, we should also consider another possibility; that ontogenetic development in the cave bear was significantly slower than in present-day bears. However, this alternative does not seem very probable, because a proper ontogenetic development has its functional importance. A long time iag in the eruption of permanent molars would mean a certain disadvantage to the species. Kratka vsebina V prispevku je opisan ontogenetski razvoj zobovja jamskega medveda {Ursus spelaeus Rosenmüller & Heinroth) vse do starosti približno štirih let, ko so živali odrastle, njihovi zobje pa so bili dokončno formirani. Proces rasti zob in zamenjave mlečnega zobovja s stalnim je pri jamskem medvedu potekal na podoben način kot

2 pri današnjem rjavem medvedu {Ursus arctos Linné). Zaporedje izraščanja zob je pri obeh vrstah enako. Primerki čeljustnic jamskega medveda, na katerih je opisan ontogenetski razvoj zobovja, so iz znanega paleolitskega najdišča Divje babe I (W Slovenija). Individualno starost teh čeljustnic smo ocenili glede na domnevo, da določena ontogenetska stopnja v razvoju zobovja jamskega medveda ustreza približno isti starosti osebka kot pri rjavem medvedu. Rast čeljusti pa je bila pri mladičih jamskega medveda vendarle bistveno hitrejša. Domnevamo tudi, da je bil pri jamskih medvedih sekundarni spolni dimorfizem izražen že v prvem letu življenja; samci so imeli večje čeljusti od samic. Glede na nekatere pokazatelje, še posebej po očitnem razhajanju med pričakovanim in dejansko ugotovljenim mortalitetnim profilom, bi morali upoštevati tudi drugo možnost; da je bil ontogenetski razvoj zobovja jamskega medveda bistveno počasnejši kot pri današnjih medvedih. Vendar, ta alternativa se ne zdi zelo verjetna, ker ima primeren ontogenetski razvoj svoj funkcionalen pomen. Dolg časovni zaostanek pri izraščanju stalnih molarjev bi za vrsto pomenil določeno pomanjkljivost. Introduction In contrast w^ith the phylogenetic evolution, the ontogenetic evolution of cave bear dentition has not been explicitly described so far, and it is generally little known to the professional public. Most researchers of the cave bear limit themselves to the statement that a certain jawbone is juvenile, which means that it belonged to a cub. Only seldom do the authors (as Rädulescu & Samson, 1959) accompany the photographs or descriptions with more detailed information on the age of the cub. The latter can be estimated by comparison with the present-day brown bear In the present work all data on teeth eruption in the brown bear are collected that are important for determining the individual age of juvenile jawbones of the cave bear and of other bear species as well. The reason for interest in the age determination of relatively rare juvenile jawbone finds is not simple curiosity. Palaeontologists and archaeologists are also interested in the behaviour of cave bears and the environment in which they lived, especially including the relationship between prehistoric man and bear From this point of view, the precise assessment of individual ages of jawbones that belonged to cubs less than one year old offers us very important information, i.e. on the season in which the animals died. By all means, the most accurate data on cave bear mortality can be achieved by the age analysis of single, isolated teeth found in tens of thousands in those Palaeolithic cave sites that had also served as typical dens of the cave bear. The individual age of particular juvenile teeth can be correctly estimated, however, only after previous study of their development and degree of formation at various ages on the set of the whole jawbones. Exactly this was the primary reason for our analysis of the jawbones of cave bear cubs from the Palaeolithic locality Divje babe I. This study was the basis for our further research on the age structure of the cave bear population from the upper part of the Pleistocene deposits (Debeljak, 1996a, 1997). The age analysis was carried out on isolated deciduous teeth d^ and permanent teeth М^. Therefore in the present paper a more detailed description of the ontogenetic development of these teeth is given.

3 Previous research Ehrenberg was the first to study the ontogenetic evolution of the cave bear He established the identity of the ontogenetic stages of dentition with those of the brown bear (1931, 640). He described the different steps in the development of mandibles, and he approximately estimated their individual ages (1931, , ). In those times only scarce data were available on tooth growth in the brown bear, so it is understandable that Ehrenberg's age estimates were imprecise. He concluded that in the well-known locality Drachenhöhle near Mixnitz many remains belong to cubs that perished from exhaustion prior to spring - less than 4 months old. According to Ehrenberg, this age-class was followed by a long gap after which appeared the group of most numerous remains of cubs about 1 year old. The supposed mortality peak at that age was explained as the consequence of complications in eruption of the last, i.e. distal molars. Ehrenberg's interpretation of the presumed absence of remains of 4-10-month-old cubs was that cave bears occupied caves only in winter, during hibernation. This opinion has prevailed among researchers until the present day. Former spectacular ideas of cave bear hunting were withdrawn by sober scientific judgement. After 30 years Ehrenberg obtained good comparative data for the present-day brown bear He attributed the age of 7 months to an almost complete skeleton of a cave bear cub from Hartlesgraben (1964, ). In the same contribution he corrected age estimates of certain mandibles made in Nevertheless, he remained convinced about the existence of the above-mentioned gap in the age composition of the cave bear remains while explaining the rare remains of 4-10-month-old cubs as an exception that confirms the rule. Musil (1965, 72) sorted the juvenile jawbones of cave bears from the Pod hradem cave according to their progressive ontogenetic development, and described them in detail. However, he did not determine their individual ages. Material and methods The ontogenetic development of cave bear dentition will be described on eleven examples of jawbones from the Palaeolithic site Divje babe I, which is located below the edge of the plateau Šebreljska planota, above the Idrijca River valley (W Slovenia). The larger part of Pleistocene deposits in the cave originate from the Middle Würm period. All fossil cave bear remains from the Divje babe I are kept in the National Museum of Slovenia in Ljubljana. General information on the locality can be found in Turk et al., 1989a, b and Turk ed., Age estimations of individual jawbones were made according to Dittrich's (1960) comprehensive data set on present-day bears, and the comparisons with juvenile jawbones of the brown bear from Slovenia. The sex was determined from the size of the teeth, especially of canines, according to the works of Koby (1949) and Kurten (1955). Some facts on the living habits of the brown bear that can also be attributed to its relative the cave bear are summarized from Macdonald ed., 1985, and Krystufek, 1991,

4 Jawbones and teeth of the cave bear - general The lower jaw of bears consists of two mandible bones (i.e. hemimandibles or mandibles). They are joined in a symphysis that never ossifies. The upper jaw is formed on each side by two paired bones: the premaxilla that bears the incisors and extends to the canine, and the upper jawbone or maxilla. These two bones are fused only in grown up individuals. In jawbones tooth sockets or alveolae appear in which tooth roots are attached. The brown bear as well as the cave bear have in both jaws on each side the following deciduous teeth: 3 incisors (dil-3), one canine (dc) and usually 3 premolars (d2-4 or p2-4). On each side of the lower jaw of the cave bear the following permanent teeth are present: three incisives (I1.3), one canine (C), usually a single premolar (P^) and three molars (M^g). In the maxilla there is one molar less. The molars have a broad masticatory surface with numerous low, rounded cusps. Early researches into the morphology of dentition and the skull revealed that the cave bear was almost exclusively vegetarian in habits (cf. Kurten, 1976). Recent isotopie analyses confirmed this hypothesis (Bocherens et al., 1994). The permanent dentition of bears was described in detail by Rode (1935). Ko by (1952) and Rädulescu and Samson, (1959) described the deciduous dentition of the cave bear For the orientation of jawbones and teeth a variety of terms is used. In this paper the following terms are met: Front - anterior - mesial. Back - posterior - distal. (This is simplified! The listed terms are not equivalent generally.) Side facing the cheeks - buccal - lateral. Towards the tongue - lingual (in the mandible) or palatal (in the maxilla). The biting (upper) surface or edge of the teeth is called occlusal. The ontogenetic development of dentition in bears (familiy Ursidae) Certain data on the eruption of permanent teeth in present-day bears can be found in the following references: Pohle, 1923; Couturier, 1954; Rausch, 1961; Marks and Erickson, However, the most detailed study of the development of deciduous dentition and its replacement by permanent teeth in the brown bear was done bydittrich (1960). He established that this process is practically the same and also synchronous in other bear species. The succession of eruption of individual teeth from the gums is presented in figure l.dittrich's other data on the ontogenetic development of dentition in bears can be summed up in the following principal points: The cubs are born toothless. The first milk teeth start erupting during the second month of life, and they come into position during the third month. The first permanent molars start piercing through the gum in the fifth month of life. The cubs are able to chew solid food only after they get the first pair of permanent molars (М^&М^). The deciduous dentition has almost no functional meaning in chewing. The deciduous teeth are shed by the end of the 15th month. The eruption of the last permanent teeth is accomplished at a mean age of one year and a half. Dittrich (1960, ) found out that the body weight in bears generally does not influence the growth and the rate of eruption of teeth. Also individual differences in this process are relatively small (see fig. 1). The disposition of deciduous and permanent teeth in the jaws of a brown bear cub is illustrated by figs. 2a and 2b.

5 Fig. 1. Ontogenetic development of dentition in the brown bear up to the age of 16 months (after Dittrich, 1960, 86, fig. 27). Individual deciduous teeth (left) are marked with a dotted pattern, and permanent teeth (right) with black. An arrow at the beginning or at the end of the symbols indicates the period in which the start of eruption of milk and permanent teeth, or shedding of milk teeth could take place. The length of the arrow therefore illustrates individual differences SI. 1. Ontogenetski razvoj zobovja pri rjavem medvedu do 16. meseca starosti (po Dittrichu, 1960, 86, si. 27). Posamezni mlečni zobje (levo) so označeni pikčasto, stalni zobje (desno) pa s črno barvo. Konica na začetku in koncu simbolov nakazuje obdobje, v katerem lahko pride do začetka izraščanja mlečnih in stalnih zob oziroma do izpada mlečnih zob. Dolžina konice torej ponazarja individualne razlike The ontogenetic development of dentition in the cave bear The first three months Osteological remains of unborn animals (fetuses) and of those cubs v^ho died at birth or shortly thereafter (neonates), are preserved only exceptionally in the Pleistocene localities. Nevertheless, such finds are not so rare in the Divje babe, because the cave served for long millenia as a den, especially to cave bear females who whelped their young in the winter months from December to February (Turk et al., 1989b; Debeljak, 1997). The thin and fragile (owing to porosity) mandibles of still unborn or newborn bears are 2-3 cm long and only around 5 mm wide. Cave bear young were born toothless. The same is true in present-day bears. Already in the first week of the brown bear's life, the germs of deciduous teeth can be

8 found in preparations of their jaws: small, hollow caps of crowns that had not yet erupted from the gums (Dittrich, 1960, 11, figs. 1, 2). This neonatal phase of the ontogenetic evolution is represented by the following example: 1. (PL 1, figs. 1, 2) - Left and right mandibles of two neonates which died in the first days after birth: For the sake of clarity the mandibles in the illustration (PI. 1, fig. 1) are joined in their natural position, although they did not belong to the same individual. In the lateral view (PI. 1, fig. 2), the body of the mandible appears semicircularly curved, which is typical for neonates. On the broken upper surface, the germs of both deciduous canines are clearly visible, and even the germ of the protoconid of the last deciduous tooth d^ or p^, as marked by some researchers. (The protoconid is the central, largest cusp on the mentioned tooth). The germs of deciduous teeth were originally enclosed in the bone. At that time the alveolae through which the growing milk teeth would later erupt were still forming. The length of the described mandible was around 4 centimeters, the same as in the newborn cubs of the brown bear Ehrenberg (1973) documented the find of an almost complete neonatus skeleton of about 10 days of age from the Austrian locality Salzofenhöhle. After various measurements and reconstruction that were made with the bones he concluded that the young of the cave bear could have been only minimally larger at birth than those of the brown bear. In a brown bear litter there are usually two or three cubs of the size of a rat, weighing only grams. They are hairless, blind and entirely helpless. They are not able to maintain their body temperature, so they can survive only with their caring mother in a den protected from the cold. In the second and third months of life the cave bear cubs most probably stayed in the den, i.e. in the cave. The female did not leave them. During the long months before the advent of spring she lived only on fat reserves collected during the late summer and autumn. For her young this was the time of lactation, of an exclusively milk diet. Meanwhile, all the deciduous teeth gradually erupted from the gums. Below them, hidden in the bone, the crowns of the permanent teeth were forming and strengthening. The eruption sequence of individual deciduous teeth can be reconstructed with the help of data on the brown bear (fig. 1). This period of ontogenic development is represented by the following two examples: 2. (PI. 2, figs. 1-3) - The mandible of a cub about months old: The crown of the first permanent М^ molar is already formed; on the x-ray photograph (PI. 2, figs. 2, 3) it is clearly visible as a thin, hollow shell encased in the jawbone. Also the first millimeters of the root wall started growing. Within the mandible the tips or crown germs of P4 and C are hidden. The permanent teeth developed in hollow chambers of the mandible with thin partitions in between. On the upper side of the mandible, in the place where М^ would later erupt, a narrow fissure is open. In front of it appear small holes; these are alveolae in which the roots of the deciduous premolars d^ and dg were anchored. They indicate that the mentioned milk teeth had already erupted from the jawbone, so the cub most probably was not younger than two months. The mandible was approximately of the same length as in an approximately four-monthold brown bear (fig. 2a), but much more robust. 3. (PI. 3, figs. 1-3) - Both halves of the mandible of a cub about months old: The furrow broadened in the place where in a month or two М^ would start erupting. Observed from above, the dark crown of М^, fragile like an egg shell, is visible through it. The upper quarter of the М^ root was already formed (PI. 3, figs. 2, 3); it is as thin as paper however Also the crowns of the permanent incisives and the germs of C and P4 teeth are concealed in the jawbone. On the upper side of the mandible the

9 alveolae of deciduous teeth are clearly visible. At this age almost all the deciduous teeth were in place, although they are not preserved here in this specimen. (The process of the eruption of milk teeth can be illustrated on an example of d4: at the age of 1-2 months the crown of d^ was enclosed in the jawbone; the crown was brown in color, dull and very fragile. Soon afterwards first the tip of the protoconid, i.e. the cusp that first breaks through the gum, became lighter colored, and later also the remaining part of the crown. In 2- to 3-month-old cubs the crown of the completely erupted d^ was already strengthened, normally bright, and with enamel of characteristic lustre. The root was, however, still hollow, and consequently rather fragile.) The mandible described is approximately of the same size as in a half-year-old brown bear. The sketch of an essentially smaller mandible of a three-month-old present-day brown bear, as published by Pohle (1923), can serve for comparison. The fourth to sixth months of life By the fourth month the bears already have all the deciduous teeth in place. The more or less formed crowns of the permanent teeth are meanwhile enclosed in the jawbones, where they gradually strengthen. For example, below the deciduous tooth d4 there is already the crown of the permanent P^ in a four-month-old cub (figs. 2a, 2b). With the progressive growth of its root the permanent tooth starts protruding and replacing the milk one. In this process, the strong protoconid of the P^ crown wedges between the two halves of the d^ root, and it gradually induces the contact resorption of the root. A similar result also occurs in the upper pair of the deciduous d'^ and permanent P"*. During the fifth month, the first permanent molars (Ml) start erupting. First the frontal part of the М^ tooth with protoconid and paraconid appears from the gums, and soon afterwards the posterior part of the crown with metaconid and talonid (Dittrich, 1960, 80-81). The dentition of the brown bear cub in fig. 2a is in the stage just described. Couturier (1954, ) published this x-ray picture of a captured cub, and attributed an age of three months to it. According to Dittrich's (1960) data it is, however, evident that this cub was in fact older, at least 4 months of age. Rädulescu and Samson (1959, 211, fig. 11) published a photograph of the mandible of an approximately 4-month-old cave bear Musil (1965, 72, Pl. 3, fig. 10) presented a specimen of the mandible that belonged to a possibly somewhat younger cub. A picture of a cave bear jawbone that would be typical for a five-month-old cub (Mj or М^ in the eruption phase) has not yet been found in the literature. In the osteological collection from the Divje babe I site there is no jawbone of a 4- to 5-month-old cub available that would be fit for publication. As a matter of fact, this is not a mere chance, since among the very numerous isolated deciduous teeth only very rare specimens were found that could be attributed to this age class. (Such are, for example, the upper or the lower d4 with strengthened root which is already somewhat resorbed at the apex. Individual cusps on the crown are rounded, and often bear tiny wear facets. Debeljak, 1996 a, 1997). Evidently the mortality of four- to five-month-old cubs in the Divje babe cave was much lower than at the younger age. The present contribution does not intend to discuss possible reasons for varying mortality rates during the year. It should only be mentioned that the fourth to sixth month of life was the time of spring. The brown bear cubs accompanied by their

10 Are our age estimates correct? Was the development of dentition in the cave bear faster, or perhaps slower than in the living brown bear? Let us discuss this problem with reference to the example of the above described mandible: It is physiologically necessary that in the sixth or at least in the seventh month the ontogenetic development results in the inclusion of the first pair of permanent molars in dentition. This is valid for the present-day brown bear, and could not have been much different for the cave bear As already mentioned, during the transition to solid plant food the most important part is assumed by the first permanent molars Ml. The mother's milk gradually ceases to suffice for growth, and the cubs have to start feeding on solid food, too. The deciduous teeth, with the exception of the last d4 premomother leave the den after the third month, in April or May, at times as late as the beginning of June. The same practice was probably true of the cave bear In addition to the milk sustenance that they get from their mother, the living bears also begin to chew solid food, starting at the fifth or sixth month. This becomes possible only after they get the first pair of permanent molars (Dittrich, 1960, 14-15). The small, pointed milk teeth are of little use for chewing. Only the new back-teeth can provide the necessary mastication surface. The seventh and eighth months of life Summer finally arrived. For the whole bear family came the time of intensive feeding. In the following months they had to collect a sufficient amount of fat, necessary for survival through the next winter The dentition of 6-8-month-old cubs were then in the following condition: 4. (PI. 4, figs. 1-5) - The mandible of a female approximately 6-7 months old: Considerable progress is evident, as compared to the specimen discussed earlier (PI. 3). The crowns of all the permanent teeth are already formed. Later they would not grow any more, but only become stronger with the secondary dentine which was gradually filling their interior The M-^ of this cub has been "in place" for a month or so, and it has more than 4/5 of the crown developed. The crown is moderately strengthened, but the root is still entirely hollow and open at the apexes (PI. 4, fig. 5). M,^ shows the first signs of wear, which indicates that the animal had already chewed hard food for some time. As a consequence of chewing numerous tiny wear facets developed, showing a characteristic glossy surface. It is obvious that the ontogenetic development of the individual teeth progressed at different rates, in accordance with the data for the brown bear presented in fig. 1. The М^ surpassed the other permanent teeth in development. It is followed by 1^, which was at that time already in place, but is not preserved here. I2 emerged through the gum shortly before the cub died. P^ and were just prior to the eruption phase. The tops of their crowns are already of lighter color. The last incisor I3 is still enclosed in the mandible, and can be seen only on the x-ray photograph. The same applies to the canine that has only the crown, but not yet the root. In this phase of growth, P^ was pushing the overlying d^ and dg deciduous teeth from the jaw. Their roots were at that time already strongly resorbed. At the latest in a month or so d^ (not preserved in our specimen) would fall out in a natural way with an entirely resorbed root. (In bears, the last deciduous premolar d^ is in use for a short period only. It takes part in chewing solid food for only about 3 months, therefore its occlusal surface is scarcely ever heavily worn away.) The size of the described mandible corresponds to a brown bear cub about one year old (PI. 9, fig. 1; PI. 10, fig. 2).

11 lars, are so tiny that they have practically no functional importance for chev^^ing hard plant food (Dittrich, 1960, 14-15, 50, 81-82). The age of the described mandible of the cave bear, therefore, cannot differ much from our estimate. In the case of a much higher age, with its chewing surface would not be "in place" soon enough to enable adequate feeding, so that the growing cub could make the best use of the summer and autumn food supply. On the other hand, the size of the mandible and the wear of that could not be caused by mere milk nutrition testify against a lower age. Perhaps this could be proven by isotopie analyses of dental tissues. In my opinion errors in the age estimates are not large, and certainly not essential. I consider that concerning the ontogenetic development of cave bear dentition, the comparison with the brown bear is appropriate. 5. (PI. 5, figs. 1, 2) - The mandible of a male bear about months old: The size of the mandible is almost equal to that of a brown bear at the age of a year and a half (PI. 16, figs. 1, 2; PI. 18, fig. 1). The anterior part of Mg protrudes out of the jawbone. The enamel becomes lighter in color during this process. In the canine the wall of the root has just started forming. The crown of Mg is enclosed in the ascending mandibular branch (i.e. ramus mandibulae), and is consequently of typical dark brown color, without lustre. A thin bone wall above Mg is broken, so it can be observed that the occlusal surface is turned lingually (inwards, towards the tongue), and it stands almost vertically with the anterior part oriented downward. Only after subsequent growing of the jawbone would the space for the two distal molars be created. (In bears, Mg rotates 90 in two directions in order to be included in the tooth row, while the jaw grows to accommodate it.) 6. (PI. 6, figs. 1-4) - The mandible of a female about months old: The jawbone belonged to a cub of a similar age as in the previous case, it was possibly only a trifle older. М^ was for a time "in place" and in use. For this reason its crown bears shallow wear facets approx. up to 3 millimeters in size. The root wall is almost entirely formed, but still open at the ends (PI. 6, fig. 4). More than half of the Mg crown already protrudes from the mandible, and about one half of the root is formed. Its wall is very thin, and the interior hollow (PI. 6, fig. 4). The М^ root is more developed and firm; the wall is about one millimeter thick. In the same phase of the ontogenetic development or at the same age, the last lower Mg molar has no root at all, and the thin, flat crown of this tooth could easily be broken in pieces (cf. Pl. 5). It is quite obvious that individual teeth of the same cub were developed to various stages, they were, therefore, of varying mechanical resistance, and consequently did not have equal possibilities to be preserved as fossils. In analyses of various isolated teeth we should take into account different degrees of taphonomic losses. Such differences occurred either as a result of the diverse shapes and sizes of individual teeth, or owing to various degree of ontogenetic development, which meant unequal firmness at a certain age. It is interesting to compare the mandible sizes in plates 4-6 (and also 11-15). Normally, the size of the bone depends upon the age of the individual. However, it is noticeable that the mandible size can be rather variable at a similar age as well - in proportion to the size of permanent teeth. The latter occupy practically the entire jawbone, as can be clearly seen on the x-ray photographs. Larger dentition also requires more space. The following conclusions can be made: 1. The cave bear had markedly larger sized dentition than the present-day brown bear It is understandable that its jaws had to be much larger and more robust in the

12 same ontogenetic phase and age. Although the cave bear cubs were as small at birth or perhaps only minimally larger than those of the brown bear, the growth of their jaws in the following months was essentially faster For instance, it was indicated that the mandible of a cave bear hardly much older than half a year (PI. 5) can be of the same size as that of a one-and-a-half-year-old brown bear Was the rest of the body also that much larger? Most probably one cannot reckon with such a rapid growth of the body mass in cave bear cubs. The female could hardly breed two or even three such big cubs during the first half-year when their diet was predominantly milk. It is more probable that the cubs had distinctly large jaws. Therefore, the jaws/body ratio must have been essentially higher than in adult cave bears, and larger than in brown bear cubs. Confirmation of this supposition is found in the only extant precisely described and measured skeleton of an approximately seven-monthold cave bear cub (finds of this kind are extremely rare!). The author of the article, Ehrenberg (1964, ), was surprised to discover that the facial part of the skull was much larger than one would expect with regard to the cranium, short trunk and weak thorax. The reconstruction revealed that at a total length of 60 cm and height of 30cm more than one third was occupied by the head (Ehrenberg, 1964, 223). It seems that the most probable reason for such a "disproportion" was the functional development of dentition that must have been as fast as with the present-day brown bear During this process the jawbone size was consequently adapted to the growing dentition. 2. Cave bear males have on the average larger teeth than females. This holds especially for the canines, according to the size of which the sex can be determined (Koby, 1949; Kurten, 1955). In cubs the wide crown base and its growing root occupy a very large part of the jawbones. It is clear in x-ray photographs that the height and thickness of the mandible body at a particular age clearly depend precisely on the size of the canines. Numerous specimens from the Divje babe indicate that in the same ontogenetic phase the mandibles of females are smaller and more gracile than those of males. Accordingly, it could be inferred that sexual dimorphism in cave bear mandibles was already manifested during the first year of life. We must also consider another possibility: that the development of permanent dentition in males lagged behind that in females. However, if such differences existed at all, they could have not been essential for the above-mentioned reason: the young must get their permanent teeth in place at a certain age to be able to feed properly. Also in present-day bears no important differences between the sexes were observed in the process of teeth growth during the first year (Dittrich, 1960; Marks &Erickson, 1966, 393). At least one example will represent the upper dentition: 7. (PI. 7, figs. 1-3) - The premaxilla and maxilla of a male approximately months old: both bones are not fused at this particular age. In present-day bears they become inseparable in adult animals - after the 4th year (Marks & Erickson, 1966, ). The М^ crown shows the first signs of wear According to the shape of the alveola, P* must have been just in the eruption phase; most of the crown probably already protruded out of the jawbone. Of the incisors only I^ is preserved. Shortly before the cub died it had probably already pierced the gum with the tip of its crown. The tooth socket of the I^ root is not damaged, which permits the observation on the completed eruption of the tooth. By contrast, the narrow aperture of the I^ alveolus proves that the eruption of this incisive had not yet started. Above this alveolus the shallow alveolus of the deciduous di^ is also preserved; at that age the tooth was still

13 in the gum, but would have dropped out of the gum in a month or so. The upper canine is hidden in the maxilla. In the x-ray photograph (PI. 7, fig. 3) it can be seen that its root had not yet started to form. The crown is hollow and is still situated much below, or better, above the rim of the upper jawbone. The deep root socket of the deciduous eye-tooth is entirely preserved and separated completely by a sohd bone wall from the developing crown of the permanent canine. Therefore we presume that the root of the deciduous canine in this phase of the ontogenetic development still did not display signs of contact resorption. Before we end the description of ontogenetic development in the seventh and eighth months of life, another curiosity should be mentioned: Among the juvenile jawbones from the Divje babe I, most specimens belong precisely to the age period being discussed, although according to other researchers (Ehrenberg, 1931, 1964; Kurten, 1958, 1976) the remains of 7-month-old cubs that died in the middle of summer must have been a real rarity. The general conviction of the last few decades is that the massively preserved remains of the cave bear in typical Pleistocene cave sites are of animals that died of exhaustion during long hibernation - in winter and mostly just before spring. However, also the analysis of isolated d^ and teeth (Debeljak, 1996 a, 1997) indicated that a remarkably high proportion of the fossil population belongs to cubs that perished presumably at an age of 6-8 months, between July and September The Divje babe as a typical cave bear site does not represent any exception. Such results about the age structure of the fossil population are therefore surprising and unexpected. We could explain them in two different ways: a) Our age estimates of the individual jawbones (and consequently also of isolated juvenile teeth) are not correct, and the most frequent age group of 7-month-old animals actually represents those cubs that died during their second hibernation. In this case, the ontogenetic development of cave bear dentition should lag as much as 6 months behind the ontogenetic development that was ascertained for present-day bears. According to the feeding habits and needs in the first year (that were described before), this seems to be almost impossible. On the other hand, in favour of the supposition above, the appearance of the first thin cementum deposit on the root of the 7-month-old cave bear cubs could be mentioned. This cementum layer was explained as the "neonatal zone" (Debeljak, 1996a, 29-30, PI ), but it is very similar to the following "winter" increments. b) The other possible interpretation of the discordance between the expected and observed mortality profile is that our age estimates are correct. Life habits and the mortality of the cave bear are then different than was thought so far. (Similarly, Musil (1965, 74-76), after measuring long bones of extremities, came to the conclusion that cave bears kept visiting the cave Pod hradem and also died there in the summer months.) Perhaps the final answer will be given by further researches into the ontogenetic development in present-day bears from different environments, and by detailed analyses of the dental cementum in bear cubs. From the ninth month to the end of the first year During the 9th or 10th month the bears finally get the second lower Mg molars. The last upper М^ molars start erupting in the ninth month. Sometimes after only the tenth month the permanent canines begin to pierce through the jawbone. Shortly

14 before the completed first year (11th or 12th month) the Mg teeth also appear through the gum. Owing to the inclined position and remarkably flat crown much time is needed before they become erupted entirely. Ehrenberg (1931, Pl. 120, fig. 6) published a photograph of a mandible of a cave bear cub to which an age of approximately 8-9 months could be ascribed. Plate 8, fig. 1 and plate 10, fig. 1 show the lower jawbone of an approximately 10- month-old present-day brown bear from Slovenia. The anterior part of the Mg is "out". The tips of the permanent canines have just appeared out of the alveola, but have not yet penetrated the gum. Lateral-distal from them the deciduous canines are attached. All the other milk teeth were shed before the tenth month. The mandible in plate 9, fig. 1 and plate 10, fig. 2 belonged to an approximately one-year-old brown bear from the Kočevje region (S Slovenia). Somewhat more than 1 cm of the canine reaches above the rim of the mandible. Beside it the deciduous canine with a worn out crown and strongly resorbed root is still in place. For a certain period during replacement of their canines the bears have the permanent and the deciduous canines in their gums at the same time. The deciduous canines are shed at an age of months only, during hibernation in the den. The above two examples of brown bear mandibles are shown here owing to the lack of an appropriate mandible for the age period being considered in the collection from the Divje babe. A well-defined gap was observed among isolated teeth (М^) as well. It appears that the Divje babe I site contains scarcely any remains of cave bears that died between the 10th month and the completed first year of life, i.e. during the first part of wintering in the cave den (Debeljak, 1996 a, 1997). However, with regard to the previous discussion, it is also possible that this "missing" age-group represents the juveniles of the next summer period. From one year to one and a half years In bears, during the period between one year and one and a half years the eruption process of permanent dentition comes to an end. The individual differences become more pronounced (Dittrich, 1960), thus the age determinations are less accurate than for the previous period. The last permanent teeth (М^, Mg and canines) are usually completely erupted in the present-day bears of one and a half years old. This period of ontogenetic development in the cave bear is represented by the following three examples. Their individual ages were estimated according to the data for present-day brown bears. Nevertheless, we should also consider the other possibility of slower ontogenetic development of dentition in the cave bear In this case, the next three mandibles could belong to two-year-olds or even older cubs that died during their third winter 8. (PI. 11, figs. 1, 2; PI. 13, fig. 1) - The mandible of a male approximately months old: The mandible size was equal to that of the adult present-day brown bear The symphyseal part, where the left and right mandibles were joined, is most probably pathologically deformed. The inflammatory alteration was possibly the result of a traumatic lesion of the jaw. About a centimeter and a half of the canine crown protrudes out of the alveolus, and more than half of the root wall is formed already. The anterior part of the chewing surface of Mg is slightly worn out. A large part of the Mg crown had not yet erupted through the gum. Its position is still inclined with respect to the tooth row.

15 During the eruption of the last lower Mg molar it sometimes happened that its crown became wedged with the adjacent Mg crown. On some adult cave bear mandibles corresponding pathologic changes can be observed that most certainly caused serious troubles to the animals. According to certain authors the irregularities in eruption of the last molars were the main, or at least a very important cause of mortality in cubs during their second winter (Ehrenberg, 1931; Abel, 1931). The investigations at the Divje babe, however, did not confirm this assumption. I believe that the difficulties mentioned could have seriously endangered cave bears mostly later in their lives. Although it was also possible that the eventual infection in a weakened cub sometimes terminated even with its death. 9. (PI. 12, figs. 1, 2; PI. 13, fig. 2) - The mandible of a female about 15 months old: Two thirds of the canine root are already formed. The crown has not yet erupted entirely. It extends for more than two centimeters beyond the margin of the alveolus. Mg is not yet "in place" or in the occlusal plane. The P^ and Mg roots are still open at the ends. The root canal of М^ is sealed and the crown "polished", with individual, 2-3 mm wide attrition facets. Wear is also evident on the anterior part of Mg. On the chewing surface of Mg, however, no traces of wear are visible. Owing to lack of space in the tooth row the individual teeth pressed one against the other (After the first year the facets started appearing at points of contact between certain teeth, and they widened and deepened with time. This is especially pronounced on the anterior wall of the Mg crown, and the posterior wall of the М^ crown. This type of wear is called approximal wear, in contrast to occlusal wear on the chewing surface). For comparison the mandible of the nearly one-and-a-half-year-old brown bear from the surroundings of Kočevje can be taken (PI. 16, figs. 1, 2; PI. 18, fig. 1). Here, too, the canines are not yet entirely erupted. However, the tooth roots are already closed, with the exception of the Mg and canines. The Mg crown has not yet fully emerged from the gums. The following individual is also in a similar ontogenetic phase: 10. (PL 14, figs, 1, 2; PL 15, figs. 1-3) - The mandible of a female cave bear approximately one and a half years old: The canine is somewhat more developed than in the brown bear just discussed. Its crown is practically entirely erupted, only about one centimeter of the root is lacking. The Mg is in place in dentition. The М^ is somewhat more worn out as in the above-discussed specimens. The surface is smoothened and covered with small wear facets. At the point of contact of М^ and Mg a well developed facet appeared on both teeth (approximal wear). On the posterior wall of the М^ crown this facet is more than 4 mm wide. The М^ root (tooth canal) has already closed during the first year At the age of around one and a half years its wall was about mm thick. At the Mg the tips of the root were still somewhat perforated. The Mg root is still widely open, and its wall is very brittle (PL 15, fig. 3). Isolated teeth have usually been defined as juvenile if the root is open, or as adult if the root is closed. In this way, certain teeth of the same individual are determined as adult, and others that lag behind in development as juvenile. The problem, however, is not only in the uncertainty of the data. The classification mentioned is inadequate first of all because a large proportion of cub teeth are incorrectly attributed to adults. The bear becomes adult around the fourth year only. The М^ root closes before the end of the first year, and the Mg and P^ at an age of one and a half years. Therefore, the closure of the root apices (or pulp canals) can be a criterion for determining adult status only in the last, most distal molars and canines. As co-author I am obliged to call attention to an error in a paper from five years

16 ago, when the ontogenetic development of dentition in bears was not yet sufficiently known to us (Turk et al., 1992). In that contribution the measurements of "adult" molars were statistically analyzed with the aim of establishing the ratio of the two sexes in the adult population of cave bears. In fact, however, the teeth examined should be attributed to older juveniles (two years, three years, possibly at most four years old). Their root was closed, and on the ground of this criterion we wrongly attributed them to adults. Further development of dentition and jawbones: Presumably in the middle or at the end of the second year of life the cave bears already had the complete permanent dentition erupted. Nevertheless the teeth continued their development. The originally hollow roots and crowns became more and more filled with dentine. Finally, in the center only a narrow canal and the pulp cavity at the passage from root to crown were left. We assume that in the cave bear during the 4th year the last tooth roots also became sealed: those of the М^, Mg and of canines. The initially explosive growth of jaws in the following years gradually slowed down. 11. (PI. 17, figs. 1, 2; PL 18, fig. 2) - The mandible of a subadult male about 4 years old: The age of this specimen was determined by counting the growth lines (i.e. increments) in dental cementum. The method is regarded as the most objective, and it is generally used for estimating the individual ages of wild animals (more in: Debeljak, 1996a, b). The above-mentioned process of dentine deposition in the interior of the teeth is already at an advanced stage, as clearly seen in the x-ray photograph (PI. 18, fig. 2). The М^ crown is the most worn out (especially the protoconid and hypoconid), Mg less and Mg very little so (PI. 17, fig. 2). At the contact of individual molars relatively deep facets developed. On the posterior wall of the М^ crown such a facet is already more than 6 mm wide. The length of the crown in this way could not be exactly measured any more. According to the x-ray picture the canine and Mg roots are not yet completely closed, although they are near to it. In present-day bears the canine root becomes closed around the 4th year (Marks & Erickson, 1966, 395, 397), i.e. during the time when they reach adulthood. The above described mandible had not yet reached its final size. The mandibles of fully grown males measure 5-10 cm more in length. The growth of jaws therefore also continued after the fourth year In the present-day black bear the head of males grows during the first 8 years, while in females the growth is terminated somewhat earlier, soon after reaching sexual maturity (Marks & Erickson, 1966, ). Most probably it was not much different in cave bears either Conclusion In this contribution the ontogenetic development of dentition in the cave bear, and bears in general, was presented. The process of replacement of deciduous dentition and the eruption of permanent teeth is generally completed by the age of one and a half years. Until then the particular ontogenetic phases are a good indicator of individual age. On the basis of the cases described and the data collected the age of juvenile cave

17 bear jawbones can be quite accurately estimated. In the future the ontogenetic development and the criteria for determining the individual age of particular (isolated) teeth that are the most numerous and most informative fossil remains of the cave bear will have to be presented in detail. Here are the principal conclusions: - The process of tooth growth and of the replacement of deciduous dentition with the permanent type took place in the cave bear in the same way, through the same and probably also synchronic ontogenetic phases as in the related present-day bears. - It is possible to infer that a certain ontogenetic phase of dentition development corresponds to approximately the same age of an individual in the cave bear, as well as in the brown bear The development of dentition in the cave bear must not have significantly lagged behind, since cubs at a certain age (in the 6th or at least in the 7th month of life) needed the first pair of permanent molars (М^ & М^) to be able to start chewing hard plant food. - This argument speaks against another possibility, that the ontogenetic development of cave bear dentition was in fact esentially slower than in present-day bears. Such a characteristic would (in certain circumstances) endanger the existence of the species. Nevertheless, this alternative still cannot and should not be simply rejected. - The size of the jawbones during the first year of life (and also later) was dependent upon the sizes of permanent teeth that were developing during that time, partly encased in the jawbones and almost completely filling them up. - The growth of jawbones in cave bear cubs was essentially faster than in the present-day brown bear Already at the age of one year their mandible was of the same size as in the adult brown bear This most certainly does not mean that the rest of the body was also larger to the same degree. - It is presumed that in the cave bear the secondary sexual dimorphism was manifested already in the first year of life; males had larger sized jaws than females. - The generally used criterion of distinguishing teeth as juvenile if their crown is open, and adult when it is closed, is not adequate for most of the cave bear teeth. The bear becomes adult at the age of around four years. The root, however, closes as early as the first year of life (after the 8th month), most of other teeth by the age of one and a half years, and only the roots of М^, Mg and canines at about four years. - In the study of juvenile jawbones from the Divje babe I site the fact cannot be overlooked that the majority of the specimens originally belonged to presumably about seven-month-old cubs that therefore died in the middle of summer This is in contradiction with the prevailing opinion that massive remains in typical cave bear localities are of animals that perished owing to exhaustion during hibernation, prior to spring. - An interesting problem and task for the future will be to prove and explain this statement with further researches and additional data. Acknowledgements I am much obliged to Ivan Turk, the leader of excavations at the Divje babe I, for the material he ceded for examination. The excavation has been carried out by the Institute for Archaeology at the Scientific Research Centre of the Slovenian Academy of Sciences and Arts. At the Clinic for surgery and small animals of the Veterinary Faculty in Ljubljana, the x-ray examination of jawbones of the cave and brown bear